A closed-loop system having a controllable variable is monitored to detect the off-nominal behavior and raise an alert when fault is detected. The faults show up in such a way that the tracking error signal is impacted and this impact manifests itself as a change in the frequency components. A filter isolates a band of frequency components of the tracking error signal that are more impacted than others by deviation caused by a fault to be detected. The filter can effectively amplify the section of the error tracking signal that contains the frequency components having impact. In addition, the filter will also have the characteristics to attenuate the impact of other frequency components. An accumulated error signal is generated from the filtered tracking error signal, compared with a predetermined fault threshold characteristic, and a fault alert is provided if a predetermined threshold is satisfied.